This invention relates to an X-ray tube, in which an anode and a cathode are coupled in a vacuum-tight manner to an evacuated envelope and, more particularly, to an X-ray tube of a spherically radiating type, which radiates X-rays uniformly in all directions at right angles to the tube axis.
The X-ray tube of this type is employed for non-destructive inspection of weldments of metal pipes or the like to check for defects and also for medical purposes, particularly dental medical purposes.
This spherically radiating type X-ray tube comprises an evacuated ceramic envelope, an anode assembly secured by a seal ring to one end of the envelope and a cathode assembly secured by a seal ring to the other end of the envelope. The anode and cathode assemblies face each other at a predetermined mutual distance. The cathode assembly includes a coil filament for emitting electrons and a focusing dimple for focusing as well as accelerating the electrons emitted from the filament. The anode assembly, on the other hand, includes a conical target, an anode block and a cylindrical X-ray radiation window member made of an X-ray transmitted material. The conical target is located at the center of the end of the target block such that it faces the filament of the cathode assembly.
In the operation of such X-ray tube, the electrons emitted from the cathode filament are accelerated by a voltage applied between the anode and cathode. The accelerated electrons impinge the conical target to form a focal spot thereon. X-rays are radiated spherically from the tip of the target.
However, when a circular focal spot on electron beam, having a uniform electron density distribution, is formed on the conical target of the above prior art X-ray tube, the temperature of the target is extremely elevated at the tip portion as compared to the peripheral portion. Therefore, when the X-ray tube is operated under a high load current, the temperature of the tip portion of the conical target is liable to exceeds the melting point of tungsten in which case the tip portion would become fused. This fusing of the tip portion will occur even if the center axis of the electron beam is accurately aligned to the tip of the conical target. This is because the target has the greatest thickness at its tip portion, i.e., the distance between the target surface and the anode block, which is made of a good thermal conductor such as copper, in the direction of the tube axis is greatest at the tip portion, and therefore the thermal conductivity of the tip portion of the target is inferior to that of the peripheral portion of the target with respect to the anode block. The tip portion of the target is thus elevated to the highest temperature.
Needless to say, there is a fear that in the prior art X-ray tube a local fusion of the target is liable to result, because the center portion of the electron beam is the area having the highest electron density of distribution. Further, if the center axis of the electron beam is not accurately aligned with the tip of the conical target, it will not obtain a uniform radiation intensity in all directions at right angles to the tube axis. To this end, there has been proposed an X-ray tube, in which the cathode assembly can be displaced relative to the anode assembly due to deformation of an intermediate deformable member, as disclosed in U.S. patent specification No. 3,714,487 to Jacob. This X-ray tube, however, is not improved at all in connection with the evasion of the fusion of the tip portion of the conical target.